Heavy duty cooling systems of the type identified above are known, for example, for use in armored vehicles, heavy trucks and the like, and are described, for example, by the German published applications Nos. 2 435 839, 2 462 475 and the German laid-open application OS No. 2 050 265.
Cooling systems employing centrifugal fans are also envisioned for use in passenger cars and trucks in order to provide adequate cooling with reduced noise generation as may be required in the future on the basis of anticipated regulations regarding noise generation. The specific noise level due to a centrifugal fan is at least 6 dB below that attainable by an axial fan of equal performance.
In the customary configuration of centrifugal fan cooling systems, the fan is located at the center of an annular radiator. Air is aspirated axially and is discharged from the fan in a generally radial or radial/tangential direction for passage through the radiator cooling fins. In order to obtain the necessary cooling capacity, it is normally required to construct the cooler, i.e., radiator, with a greater axial length than the axial width of the air outlet orifice of the concentric centrifugal fan. This discrepancy is due to the fact that the maximum axial length of the fan blades is determined by the maximum permissible delay occurring between the passage of air through the narrowest cross section of the inlet aperture and the inlet area of the fan blades. This delay must not exceed a given limit beyond which there would occur a flow separation within the fan wheel. Accordingly, the fan wheel outlet axial length cannot generally be made equal to the axial length of the annular radiator.
For example, in known cooling systems of this type of construction, the ratio of the radiator axial length to the axial length of the blower outlet orifice is approximately 2.0-3.3 (see for example U.S. Pat. No. 3,698,473 or the German patent No. PS 1 576 705). Furthermore, the radial distance between the air outlet of the fan and the inlet of the radiator must be kept relatively small so as to save space. Accordingly, the air flow is concentrated in the middle of the radiator, i.e., in the general vicinity of the blower outlet but is substantially reduced in the lateral regions of the annular radiator.
In order to overcome this disadvantage, it has been proposed to dispose conical guide rings between the fan and the radiator, for example as described in the U.S. Pat. No. 3,698,473 or the German Pat. No. 1 576 705, as well as in the German laid-open application No. 2 050 265. This construction defines a number of radial diffusors which generally enlarge the blower outlet orifice to the axial length of the cooler inlet side. However, this construction is very complicated and expensive.
It has further been proposed in the German published application No. 2 462 475 to set a given ratio of blower axial length to radiator axial length as well as a ratio of blower diameter to radiator diameter so as to optimize the velocity distribution of the air within the radiator. However, measurements made on cooling systems which obey these conditions have shown that the results are still less than satisfactory.